1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
In the field of magnetic recording media used in a HDD (Hard Disk Drive) or the like, the recording density continues to improve at a considerable rate. The recording density is recently increasing at a rate of over approximately 1.5 times per year. One of key technologies for improving the recording density includes a technique that controls a sliding contact characteristic between a magnetic head and the magnetic recording medium.
On the other hand, a Winchester type HDD employs a CSS (Contact Start Stop) system in which a basic operation from a start to stop of the magnetic head includes sliding contact, floating, and sliding contact with respect to the magnetic recording medium. The sliding contact of the magnetic head with respect to the magnetic recording medium is unavoidable, including a case in which an unintentional sliding contact occurs.
For this reason, problems related to tribology between the magnetic head and the magnetic recording medium are the technical problems to be solved inherently. Hence, improving a friction resistance and a sliding contact resistance of the magnetic recording medium by forming a protection layer on a magnetic layer of the magnetic recording medium is one of popular methods of securing a reliability of the magnetic recording medium.
Various materials have been proposed for the protection layer. From viewpoints of easy deposition, durability, or the like, carbon is mainly used for the protection layer. Hardness, density, dynamic coefficient of friction, or the like of the protection layer are important parameters in that these parameters truly affect the CSS characteristic of the magnetic recording medium.
On the other hand, in order to improve the recording density of the magnetic recording medium or improve a read and write speed with respect to the magnetic recording medium, it is preferable to reduce a flying height of the magnetic head, or increase a rotational speed of the magnetic recording medium. Accordingly, the sliding contact resistance and a flatness of the protection layer are required to cope with the unintentional contact or the like with the magnetic head. In addition, the protection layer is preferably thin, and is required to be 30 Å or less, for example, in order to reduce a spacing loss between the magnetic recording medium and the magnetic head and increase the recording density.
In addition, the protection layer is required to have corrosion resistance in order to prevent corrosion generated by environmental substance diffusing onto the magnetic layer of the magnetic recording medium.
A carbon layer that is used for the protection layer of the magnetic recording medium is formed by sputtering, CVD (Chemical Vapor Deposition), ion beam enhanced deposition, or the like. The carbon layer that is formed by the sputtering may have an insufficient durability in a case in which the thickness is 100 Å or less. In addition, the carbon layer that is formed by the CVD easily becomes crystalline and has a poor surface smoothness. Further, in a case in which the carbon layer that is formed by the CVD is thin, a coverage of the surface of the magnetic recording medium deteriorates, and the corrosion of the magnetic recording medium may occur. On the other hand, the ion beam enhanced deposition can form a dense carbon layer having a high hardness and a high surface smoothness, when compared to the sputtering and the CVD.
Known hard carbon layers that may be used for the protection layer include a diamond layer and a DLC (Diamond-Like Carbon) layer. In general, the diamond layer is a crystal layer having a diamond bond of approximately 100%. The DLC layer may be a hard carbon layer that is amorphous, and is thus sometimes referred to as an amorphous carbon layer. The carbon layer that is used for the protection layer of the magnetic recording medium is required to have a high surface smoothness, and for this reason, the crystalline diamond layer is not used in general, and the DLC layer (amorphous carbon layer) is used. Particularly due to the high surface smoothness obtained thereby, a hydrogenated DLC layer (hydrogen-containing DLC layer (amorphous carbon layer)) is preferably used for the protection layer of the magnetic recording medium.
For example, Japanese Laid-Open Patent Publication No. 2013-101742 proposes a magnetic recording medium having a plurality of layers including a plurality of magnetic particles and graphitic carbon formed on the magnetic particles. The graphitic carbon may take various forms, including graphite, graphene (single atomic layer of graphite), nanotube (graphene sheet that is wound to a cylindrical shape), fullerene (graphene sheet that is wound to a closed shape such as a sphere or the like), or the like.
For example, Japanese Laid-Open Patent Publication No. 2013-536141 proposes a FePt/graphene structure for use in hard disks.
Studies to improve the protection layer of the magnetic recording medium continue. Presently, a hydrogenated amorphous carbon layer is popularly used for the protection layer of the magnetic recording medium. The hydrogenated amorphous carbon layer has a high surface smoothness and a relatively high hardness. On the other hand, because the hydrogenated amorphous carbon layer has an amorphous structure, a layer characteristic thereof has a variance, and the friction resistance, the sliding contact resistance, and the corrosion resistance may vary depending on deposition conditions. In addition, since the surface of the hydrogenated amorphous carbon layer is basically water repellent, it is difficult to provide a lubricant coating on the hydrogenated amorphous carbon layer. For this reason, the surface of the hydrogenated amorphous carbon layer needs to be modified by nitridation, oxidation, or the like, to thereby make it difficult to reduce the thickness of the protection layer.